mempool: add CheckSpend method
mempool: add CheckSpend method

This commit adds a new method CheckSpend to the mempool, which takes an
outpoint and returns any transaction in the mempool that spends this
outpoint.

btcsuite/btcd,dajohi/btcd,wallclockbuilder/btcd,tb00/btcd,tb00/btcd,ltcsuite/ltcd,dajohi/btcd,dajohi/btcd,davecgh/btcd,ppcsuite/ppcd,wallclockbuilder/btcd,btcsuite/btcd,aakselrod/btcd,davecgh/btcd,ppcsuite/ppcd,aakselrod/btcd,ltcsuite/ltcd